It didn't occur to me till the last time I saw it (less than a week ago), but showing a smudge on his forehead TWICE sounds like they were "riffing" on Macbeth, where Lady Macbeth can't seem to lose the spot on her hand. Maybe I'm all wrong about that, but if I'm not, it would be a second episode with a Macbeth connection, after Dagger of the Mind.
I recently watched an episode of the British crime series "Father Brown" which features a crime solving Catholic priest in a small English village in the late 1940s, early 1950s.
In this episode the supposed murder victim had actually framed someone by committing suicide. The method? Tieing the gun to a large stone and dangling the rope down the chimney. After the victim fired the gun he let go of it and the weight of the stone slid down the roof above pulling the weapon up the chimney. The stone and rope were obscured by the angle of the roof.
I thought to myself, where have I heard that idea before?